Electro-optic materials, such as lithium niobate (LNB) and KTP, have refractive indices which vary according to the magnitude and direction of applied electric field. Waveguide devices based on such materials are potentially useful for optical fibre communication and signal processing systems. Typically such devices are required to operate with light of wavelengths in the range 0.6 to 1.6 .mu.m, and in particular with light in the range 1.3 to 1.6 .mu.m.
There are two basic device types: directional couplers; and Mach-Zehnder (MZ) interferometers. The first of these utilises the electro-optic effect to control the coupling between a pair of adjacent waveguides. By controlling their refractive indices it is possible to couple light from one waveguide to the other or vice versa. In an MZ interferometer an input waveguide is coupled to an output waveguide by a pair of waveguide arms. Each arm has an associated electrode by means of which it is possible to control the refractive indices of, and hence the velocity of propagation in, the two arms independently. It is therefore possible, by controlling the applied electric fields, to produce phase differences between signals travelling in the two arms resulting in constructive or destructive interference when they are combined. Thus it is possible to amplitude modulate input optical signals according to the voltage difference between the electrodes. Unfortunately, materials such as LNB, which exhibit the electro-optic effect tend also to be pyroelectric: electric fields are produced within the material as the result of a temperature change. With some materials, notably z-cut LNB, the pyroelectric effect is so strong that a temperature change of a degree or less may be sufficient to produce an electric field comparable to that applied to produce switching of states in a directional coupler or MZ interferometer made of the material. Such electric fields strongly affect the optical states of the devices. Consequently it is necessary, with materials such as z-cut LNB which exhibit a strong pyroelectric effect, to provide very precise temperature control if reliable and repeatable performance is to be achieved from electro-optic waveguide devices based on such materials. Clearly the need to provide precise temperature control is a disadvantage and a disincentive to the use of such materials. With z-cut LNB this disincentive is so strong that despite its stronger electro-optic effect, which would make possible the use of lower operating voltages and shorter devices, the material is eschewed in favouf of x-cut LNB, which is less strongly pyroelectric, despite the latter's inferior electro-optic properties. Unfortunately, electro-optic devices made from x-cut LNB, unlike those made from z-cut LNB, require complex electrode structures which are generally incompatible with high speed operation.
The problems of thermal instability are particularly severe in devices in which there is a non-symmetrical arrangement of electrodes. Examples of devices with non-symmetrical electrode arrangements include directional couplers and MZ interferometers having travelling-wave electrodes. The use of travelling-wave electrodes potentially enables the production of devices capable of very high speed operation (typically switchable at gigabit rates). In such devices the electrode arrangement consists of a first electrode, overlying a first waveguide arm of the device and configured as a transmission line, generally in the form of a narrow strip, and a second electrode, the ground electrode, overlying a second waveguide arm of the device, and generally much more extensive than the first electrode.
The problem of the thermal instability of devices made from z-cut LNB has been investigated, see for example the paper by Skeath et al, Appl. Phys. Lett., 49 (19), Nov. 10, 1986, pp 1221-1223, and that by Gee et al, Appl. Phys. Lett., 47 (3), Aug. 1, 1985, pp 211-213, but as yet no one appears to have devised a workable solution which would enable the system application of z-cut LNB without very precise temperature control, particularly in the case that the electrodes are non-symmetrically disposed over the waveguide branches.